Knowledge Checks

Reliable data transfer protocol mechanisms.

Consider the purposes/goals/use of different reliable data transfer protocol mechanisms. For the given purpose/goal/use match it to the RDT mechanism that is used to implement the given purpose/goal/use.

Question List:

Lets the sender know that a packet was NOT received correctly at the receiver.

Used by sender or receiver to detect bits flipped during a packet’s transmission.

Allows for duplicate detection at receiver.

Lets the sender know that a packet was received correctly at the receiver.

Allows the receiver to eventually receive a packet that was corrupted or lost in an earlier transmission.

Answer List:

ACK
Sequence numbers
Checksum
NAK
Retransmission

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1/14

The rdt 2.0 protocol.

Consider the rdt 2.0 sender and receiver shown below, with FSM transitions at the sender labeled S1, S2, and S3; and receiver transitions labeled R1 and R2.

Which of the following sequences of transitions could possibly occur as a result of an initial rdt_send() call at the sender, and possible later message corruption and subsequent error recovery.

S1, R1, S2, R2,S3

S1, R2, S3

S1, R2, S2

S1, S2, S3

S1, R1, S2

S1, R1, S3

S1, R1, S2, R1, S3

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2/14

The rdt 2.1 protocol (a).

Consider the rdt2.1 sender and receiver FSMs shown below, with labeled transitions S1 through S6 at the sender, and transitions R1 through R6 at the receiver. The sender and receiver start in the “Wait for call 0 from above” and “Wait for 0 from below” states, respectively.

Suppose that no channel errors occur. A sequence of interleaved sender and receiver transitions is given below. Transitions S1 and S4 are already provided. Choose the sender or receiver transition for the unlabeled transitions x₁, x₂, x₃, and x₄ below to indicate the time-ordered sequence of transitions (interleaved sender and receiver transitions) that will result in two messages being delivered at the receiver, with the sender and receiver returning to their initial states (again, given that no channel errors occur).

S1, x₁, x₂, S4, x₃, x₄

Question List:

transition x₁

transition x₂

transition x₃

transition x₄

Answer List:

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3/14

The rdt 2.1 protocol (b).

Suppose that the initial message transmission by the sender is corrupted, but that no other message transmissions are corrupted. Match the unlabeled transitions x₁, x₂, x₃, x₄, x₅ in the time-ordered sequence of transitions below (interleaved sender and receiver transitions) that will occur following the initial S1 transition (which is corrupted), that will result in two messages being delivered at the receiver, with the sender and receiver returning to their initial states (again, given that the initial message transmission by the sender is corrupted). Note that transitions S1, S4, and S6 are already provided below.

S1 (message corrupted), x₁, x₂, x₃, x₄, S4, x₅, S6.

Question List:

transition x₁

transition x₂

transition x₃

transition x₄

transition x₅

Answer List:

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4/14

The rdt 2.1 protocol (c).

Suppose that the first packet from the sender is correctly received at the receiver but that ACK message sent from receiver-to-sender is corrupted; all other messages (before or after that ACK) are transmitted error-free. Match the unlabeled transitions x₁, x₂, x₃, x₄, x₅ in the time-ordered sequence of transitions below (interleaved sender and receiver transitions) that will occur following the initial S1 transition, which is followed by a corrupted ACK transmission, that will result in a message being delivered at the receiver, with the sender and receiver returning to their initial states. Note that some transitions are already provided below.

S1, x_{1 (ACK corrupted)}, x₂, x₃, x₄, S4, x₅, S6.

Question List:

transition x₁

transition x₂

transition x₃

transition x₄

transition x₅

Answer List:

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5/14

Cumulative ACK.

What is meant by a cumulative acknowledgment, ACK(n)?

A cumulative ACK(n) allows the receiver to let the sender know that it has not received any packets with a new sequence number since the last cumulative ACK(n) was sent.

A cumulative ACK(n) allows the receiver to let the sender know that it has not yet received an ACK for packet with sequence number n.

A cumulative ACK(n) acks all packets with a sequence number up to and including n as being received.

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6/14

Stop-and-wait: channel utilization.

Suppose a packet is 10K bits long, the channel transmission rate connecting a sender and receiver is 10 Mbps, and the round-trip propagation delay is 10 ms. What is the maximum channel utilization of a stop-and-wait protocol for this channel?

.01

10.0

.001

1.0

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7/14

Channel utilization with pipelining.

Suppose a packet is 10K bits long, the channel transmission rate connecting a sender and receiver is 10 Mbps, and the round-trip propagation delay is 10 ms. What is the channel utilization of a pipelined protocol with an arbitrarily high level of pipelining for this channel?

0.001

10.0

0.01

0.1

1.0

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8/14

Channel utilization with pipelining (more).

Suppose a packet is 10K bits long, the channel transmission rate connecting a sender and receiver is 10 Mbps, and the round-trip propagation delay is 10 ms. How many packets can the sender transmit before it starts receiving acknowledgments back?

100

10,000

1000

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9/14

Pipelining.

Which of the following statements about pipelining are true? One or more statements may be true.

With pipelining, a packet is only retransmitted if that packet, or its ACK, has been lost.

With pipelining, a receiver will have to send fewer acknowledgments as the degree of pipelining increases

With a pipelined sender, there may be transmitted packets “in flight” – propagating through the channel – packets that the sender has sent but that the receiver has not yet received.

A pipelined sender can have transmitted multiple packets for which the sender has yet to receive an ACK from the receiver.

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10/14

Packet buffering in Go-Back-N.

What are some reasons for discarding received-but- out-of-sequence packets at the receiver in GBN? Indicate one or more of the following statements that are correct.

Discarding an out of sequence packet will really force the sender to retransmit.

The sender will resend that packet in any case.

If some packets are in error, then its likely that other packets are in error as well.

The implementation at the receiver is simpler.

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11/14

Packet buffering in Go-Back-N (more).

What are some reasons for not discarding received-but- out-of-sequence packets at the receiver in GBN? Indicate one or more of the following statements that are correct.

By not discarding, the receiver can implicitly let the sender know that it (the sender) does not necessarily have to retransmit that packet.

Complex protocols are always better.

Even though that packet will be retransmitted, its next retransmission could be corrupted, so don’t discard a perfectly well-received packet, silly!

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12/14

Receiver operation in Selective Repeat.

In the SR receiver window (see diagram below, taken from PPT slides and video), why haven’t the red packets been delivered yet? Check the one or more reasons below that apply.

There is a packet with a higher sequence number than any of the red packets that has yet to be received, so in-order delivery of data in the red packets to the application layer is not yet possible.

Red packets have a lower delivery priority up to the application.

There is a packet with a lower sequence number than any of the red packets that has yet to be received, so in-order delivery of data in the red packets up to the application layer is not possible.

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13/14

Receiver operation in Selective Repeat (more).

In SR, why does the receiver have to acknowledge packets with sequence numbers that are less than (and to the left of) those in its window, which starts at rcv_base.

Actually, this ACK retransmission can be ignored and the protocol will still function correctly, but its performance won’t be as good.

Because, at the time of the data packet arrival at the receiver, the sender has definitely still not received an ACK for that packet.

Because the sender may not have received an ACK for that packet yet.

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14/14

Knowledge checks

Reliable data transfer protocol mechanisms.

Question List:

Answer List:

The rdt 2.0 protocol.

The rdt 2.1 protocol (a).

Question List:

Answer List:

The rdt 2.1 protocol (b).

Question List:

Answer List:

The rdt 2.1 protocol (c).

Question List:

Answer List:

Cumulative ACK.

Stop-and-wait: channel utilization.

Channel utilization with pipelining.

Channel utilization with pipelining (more).

Pipelining.

Packet buffering in Go-Back-N.

Packet buffering in Go-Back-N (more).

Receiver operation in Selective Repeat.

Receiver operation in Selective Repeat (more).